Light emitting diodes (LED) have hitherto been developed for several decades, red, green and yellow light emitting diodes have been successfully developed and have reached the level of commercialization. Blue/green light emitting diodes with ultra-high brightness have been recently investigated in institutions all over the world. III-nitride semiconductor light emitting materials show themselves beyond many semiconductor light emitting materials of green-ultraviolet band since they are advantaged in direct energy gap, strong chemical bonds and high beat conductivity. Nitrides, including aluminum nitride (AlN), gallium nitride (GaN), indium nitride (InN) and the like, can be used for emitting light almost over the range of visual light and even extending to ultraviolet field. Thus, the nitrides have been recently used in various high-tech products such as elements capable of emitting light over the range from blue/green light to ultraviolet light, high power and high temperature electrical elements, large-sized fully-colored electrical signboards, traffic lights, backlight sources of liquid crystal display, special emitting light sources, semiconductor laser reading head of digital video discs (DVD) and the like.
Now, commercialized blue/green light emitting diodes in use are mainly made of III-nitrides and V-nitrides but the photoelectric effect is insufficient due to the polarity caused by epitaxial direction and structure of the materials. Generally, when polar c-plane III-nitride is employed, a built-in electric field occurs in moving direction of carriers due to asymmetric atomic charges to further generate quantum confined stark effect (QCSE) which deflects quantum trap energy gap structure to reduce overlap odds of electron wave function and hole wave function and further to cause decrease of light emitting efficiency and red shift of the spectrum. In addition to increase of photoelectric effect to enhance light emitting efficiency of an element non-polar III-nitrides and V-nitrides also facilitate application of the light emitting element since the light emitted by the non-polar material has the performance of polarized light. Generally, non-poly GaN is formed on a sapphire (Al2O3) substrate and then an LED or a laser diode is manufactured.
FIG. 1A and FIG. 1B show a manufacture method of a general non-polar a-plane {11-20} GaN. Referring to FIG. 1A, a r-plane {11-102} sapphire substrate (10) is placed in a metal organic chemical vapor deposition (MOCVD) device and annealed at high temperature, then a low-temperature aluminum indium nitride (AlInN) layer (11) is formed at 400-900° C. As shown in FIG. 1B, non-polar a-plane {11-20} GaN (12) is formed on the AlInN layer (11) by MOCVD process, wherein V/III ratio of the gallium source is about 770-2,310, and epitaxial pressure of the nitrogen source is about 0.5 atm (or higher) and epitaxial temperature is 1100° C.
However, difference between lattice constants of layers causes lattice mismatch to form dislocation during formation of the layers, thus, general manufacture method of non-polar a-plane {11-20} GaN cannot effectively reduce dislocation density which risks generation of holes on surface of the GaN to further cause problems of the GaN surface such as roughness or cracking.
Thus, in order to overcome the problems above-mentioned, it is desired to reduce dislocation density generated during forming GaN on a sapphire substrate, thereby forming GaN with flat surface and high crystal quality.